Typical L2TP networking application
Figure 88: VPDN built by using L2TP
As shown in Figure 88, a VPDN built by using L2TP comprises the following components:
Remote system—A remote system is usually a remote user's host or a remote branch's routing device that needs to access the VPDN network.
LAC—An L2TP access concentrator (LAC) is a device with PPP and L2TP capabilities. It is usually a NAS located at a local ISP, which provides access services mainly for PPP users.
An LAC is an endpoint of an L2TP tunnel and lies between an LNS and a remote system. It encapsulates packets received from a remote system using L2TP and then sends the resulting packets to the LNS. It de-encapsulates packets received from the LNS and then sends the resulting packets to the intended remote system.
Usually, a PPP link is used in a VPDN application.
LNS—An L2TP network server (LNS) functions as both the L2TP server and the PPP end system. It is usually an edge device on an enterprise network.
An LNS is the other endpoint of an L2TP tunnel and is a peer to the LAC. It is the logical termination point of a PPP session tunneled by the LAC. The L2TP extends the termination point of a PPP session from a NAS to an LNS, logically.